1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image.
2. Description of the Prior Art
An image forming apparatus capable of forming (or copying) images on both sides of a record medium has been known. In such an apparatus, a single side mode in which an image is copied on one side of the record medium and a double side mode in which images are copied on the both sides are selectively used to copy the images. In the prior art apparatus of this type, it is possible to interrupt the double side mode by the single side mode by depressing an interruption key, but it is not possible to interrupt the single side mode by the double side mode. Thus, an operator has to wait for the completion of the current copying operation.
In the prior art apparatus of this type, if a copying operation for a back side is cancelled during the execution of the double side mode copying operation, a record medium having an image copied on one side thereof is left in an intermediate tray of the apparatus and the operator must take out the record medium from the intermediate tray.
When a sorter is used in combination with the apparatus, the record media are sorted without images being copied on the back sides and hence the pages are disordered.
If the record media are double-fed during the back side copying operation and hence the record media in the intermediate tray are exhausted before a preset number of media are handled, the copying operation is stopped as the jam has occurred or an optical system is scanned while no record medium is present.
The copying machine consumes a high power during the copying operation. In the copying machine which uses a home plug socket (100 V, 15 A), a drive current must always be maintained below a predetermined level. However, since a fixer heater in the copying machine consumes a high power (e.g. 800 W), the current consumed significantly increases when a deck which accommodates a large number of record papers (e.g. 2000 papers) is lifted up. As a result, the current significantly exceeds the current rating of the plug socket.
A transferred image must always be fixed in a predetermined temperature range (e.g. around 180.degree. C). However, when a large number of images are continuously fixed, the heat is absorbed by the record papers and the temperature of the fixer gradually falls. If the temperature of the fixer falls below a predetermined temperature (e.g. 150.degree. C.), the copying operation must be stopped until the temperature of the fixer rises. As a result, a throughput is lowered.
In the image forming apparatus such as the copying machine, the current capacity is limited due to the requirement for the plug socket. Accordingly, it is usually impossible to increase the heater drive current when the temperature of the fixer drops.
In such apparatus, independently operated microprocessors such as a microprocessor exclusively used to control a sequence and a microprocessor for managing an overall copying operation are used. With such a construction, however, the number of input/output ports of the sequence control processor is very large and a large volume of wirings are required for the apparatus.
In a system having a plurality of processors, necessary communication control is carried out by using microprocessors in the respective units of the system. Those microprocessors require lines for transmitting and receiving request signals. The communication is done in one way, that is, only the unit which issued the transmission request transmits a data and other units receive the data. As a result, the communication procedures are complex and it is difficult to reduce the communication time. Further, a large volume of lines must be connected among the processors.
When a serial communication is to be carried out among a plurality of processors, a communication request right is imparted to each of the processors and a complex communication control is carried out. As a result, the communication control is complex particularly when the communication requests compete.